The invention relates to a projection television system comprising a cathode-ray tube having a face plate, a display screen coated with a blue cathodoluminescing phosphor containing ZnS:Ag and an interference filter arranged between the face plate and the display screen.
The invention also relates to a cathode-ray tube suitable for use in a projection television system.
A projection television system of the type defined in the opening paragraph and a cathode-ray tube mentioned in the second paragraph are known from Journal of the Electrochemical Society, volume 135, No. 2, pages 858-862.
Important parameters for a projection television system are the chromaticity coordinates of the primary colours red, green and blue and the maximum achievable spectral energy distribution for what is commonly denoted the white-D-point. The chromaticity coordinates are important since only colours within a triangle in the CIE (International Commission on Illumination) 1931 diagram, whose vertices are formed by the chromaticity points of the primary colours, can be displayed by a television system. In practice, the aim is for the chromaticity coordinates of the primary colours to satisfy standards, for example the EBU standards. These standards are inter alia mentioned in "E.B.U. standard for chromaticity tolerance for studio monitors, E.B.U. Technical Centre, Tech. 3213-E, Brussels 1975". In practice, for the cathode-ray tube emitting blue light the y-coordinate of the emitted light is generally of greater importance than the x-coordinate.
The white-D-point is a point in the CIE 1931 chromaticity coordinates diagram having the chromaticity coordinates x=0.313 and y=0.329. The maximum luminance in this point is determined by the following parameter, denoted Q in the remainder of this description, of the blue light emitting cathode-ray tube: ##EQU1## Therein
.eta.CR is the energy efficiency of the blue luminescing phosphor,
L is the lumen equivalent of the spectral emission of the blue light, and
y is the y-coordinate of the chromaticity coordinates of the blue light.
A known blue phosphor is ZnS:Ag. This phosphor has a wide-band emission spectrum having an emission peak at approximately 448 nm. The y-coordinate of the chromaticity of the emission spectrum of ZnS:Ag (y=0.054) satisfies the EBU standards for the y-coordinate (0.053&lt;y&lt;0.072.)
In the J. Electrochem. Soc. reference it is described that by arranging an interference filter between the display screen and the face plate the Q can be increased, but that for ZnS:Ag only a slight increase thereof is possible without decreasing the y-coordinate to less than 0.053.